memcg: update memcg_has_children() to use css_next_child()

Currently, memcg_has_children() and mem_cgroup_hierarchy_write()
directly test cgroup->children for list emptiness.  It's semantically
correct in traditional hierarchies as it actually wants to test for
any children dead or alive; however, cgroup->children is not a
published field and scheduled to go away.

This patch moves out .use_hierarchy test out of memcg_has_children()
and updates it to use css_next_child() to test whether there exists
any children.  With .use_hierarchy test moved out, it can also be used
by mem_cgroup_hierarchy_write().

A side note: As .use_hierarchy is going away, it doesn't really matter
but I'm not sure about how it's used in __memcg_activate_kmem().  The
condition tested by memcg_has_children() is mushy when seen from
userland as its result is affected by dead csses which aren't visible
from userland.  I think the rule would be a lot clearer if we have a
dedicated "freshly minted" flag which gets cleared when the first task
is migrated into it or the first child is created and then gate
activation with that.

v2: Added comment noting that testing use_hierarchy is the
    responsibility of the callers of memcg_has_children() as suggested
    by Michal Hocko.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>

1 files changed, 21 insertions, 10 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6144a8e7283f..b6f91d61b3af 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4834,18 +4834,28 @@ static void mem_cgroup_reparent_charges(struct mem_cgroup *memcg)
         } while (usage > 0);
 }
 
+/*
+ * Test whether @memcg has children, dead or alive.  Note that this
+ * function doesn't care whether @memcg has use_hierarchy enabled and
+ * returns %true if there are child csses according to the cgroup
+ * hierarchy.  Testing use_hierarchy is the caller's responsiblity.
+ */
 static inline bool memcg_has_children(struct mem_cgroup *memcg)
 {
-        lockdep_assert_held(&memcg_create_mutex);
+        bool ret;
+
         /*
-         * The lock does not prevent addition or deletion to the list
-         * of children, but it prevents a new child from being
-         * initialized based on this parent in css_online(), so it's
-         * enough to decide whether hierarchically inherited
-         * attributes can still be changed or not.
+         * The lock does not prevent addition or deletion of children, but
+         * it prevents a new child from being initialized based on this
+         * parent in css_online(), so it's enough to decide whether
+         * hierarchically inherited attributes can still be changed or not.
          */
-        return memcg->use_hierarchy &&
-                !list_empty(&memcg->css.cgroup->children);
+        lockdep_assert_held(&memcg_create_mutex);
+
+        rcu_read_lock();
+        ret = css_next_child(NULL, &memcg->css);
+        rcu_read_unlock();
+        return ret;
 }
 
 /*
@@ -4919,7 +4929,7 @@ static int mem_cgroup_hierarchy_write(struct cgroup_subsys_state *css,
          */
         if ((!parent_memcg || !parent_memcg->use_hierarchy) &&
                                 (val == 1 || val == 0)) {
-                if (list_empty(&memcg->css.cgroup->children))
+                if (!memcg_has_children(memcg))
                         memcg->use_hierarchy = val;
                 else
                         retval = -EBUSY;
@@ -5036,7 +5046,8 @@ static int __memcg_activate_kmem(struct mem_cgroup *memcg,
          * of course permitted.
          */
         mutex_lock(&memcg_create_mutex);
-        if (cgroup_has_tasks(memcg->css.cgroup) || memcg_has_children(memcg))
+        if (cgroup_has_tasks(memcg->css.cgroup) ||
+            (memcg->use_hierarchy && memcg_has_children(memcg)))
                 err = -EBUSY;
         mutex_unlock(&memcg_create_mutex);
         if (err)